Dielectric drying of materials



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ATTOR/VEKSI H. J. CAMERON DIELECTRIC DRYING OF MATERIALS June 26, 1962 Filed Oct. 13, 1958 5 Sheets-Sheet 2 BgUGH J CAME/POM June 26, 1962 H. J. CAMERON DIELECTRIC DRYING OF MATERIALS Filed Oct. 13, 1958 5 Sheets-Sheet 3 TPA NSFOIE'MEIE 6'16 fiPE us/vcr .4 7

Sou/Pas 13f? 772 INVENTOR.

Arrom/Er June 26, 1962 H. J. CAMERON DIELECTRIC DRYING OF MATERIALS 5 Sheets-Sheet 4 Filed- Oct. 13, 1958 x 41th .0

.N mm m m M WM m 1 #4 J a HUWW fi June 26, 1962 H. J. CAMERON 3,041,435

DIELECTRIC DRYING OF MATERIALS Filed Oct. 15, 1958 5 Sheets-Sheet 5 INVENTOR. HUGH JCAMEEOIV.

ATIURNEYJ.

United States Patent 3,041,435 DIELECTRIC DRYING OF MATERIALS Hugh J. Cameron, New Rochelle, N.Y., assignor, by

mesne assignments, to American Machine & Foundry Company, New York, N.Y., a corporation of New Jersey Filed Oct. 13, 1958, Ser. No. 766,959 16 Claims. (Cl. 219-1059) This invention relates to radio frequency heating apparatus and particularly to apparatus for separating volatile liquids from other material by dielectrically heating the material to cause evaporation of the liquid therefrom. The invention is particularly adapted for the removal of water from porous or absorbent solids.

The invention will be herein described in a form particularly adapted for the drying of so-called cakes of rayon yarn, but it will be understood that the invention in various of its aspects is Well adapted for the heating of various materials and for the drying of other materials, such as textiles, wood, sponge rubber and the like, paper and pulp materials and the baking of articles such as foundry cores.

The drying of materials by dielectric heating involves the reducing of the water content of the material by the internal generation of heat therein, produced by subjecting such material to an electrical field maintained between a pair of spaced electrodes and having a frequency of at least one megacycle per second. In order that such high frequency field may be sufficiently effective for the purpose, it is in practice necessary to apply to the electrodes a potential high enough, e.g. several thousand volts, such as to be liable to generate arcs and cause dielectric breakdown of the air space about the electrodes. The presence of water in the form of mist or as droplets on any conducting surface in the vicinity, greatly increases the liability of arc formations and destructive electrical discharges.

In conventional dielectric heating apparatus, it is frequently desirable that the material to be heated be moved lengthwise of and between a pair of relatively long electrodes which are connected to a high frequency, electrical energy source. The electrodes act as capacitor plates and transfer energy to the material in proportion to the voltage between the plates, the frequency of the energy, the properties of the material, the physical relationship between the plates and the material, the configuration of the plates, etc.

At the relatively high frequencies employed for dielectric heating of materials, the wave length is such that the conductors normally employed, and particularly the electrodes and the lines which connect the electrodes to the source, have a length which is an appreciable fraction of a wave length, and hence, the voltage may vary from point-to-point along the conductors unless care is taken to maintain correct impedance matching. Furthermore, a slight change in impedance at any point along the conductors may produce a relatively large change in electrical potential at other pointsbecause the conductors act as transformers at such frequencies. Accordingly, with prior art dielectric heating equipment, it has been ditiicult to obtain uniform drying of the material due to the fact that the dielectric properties of the material 3,041,435 Patented June 26, 1962 ice vary from piece-to-piece or cake-to-cake and with drying thereof and that the loading, and hence heating energy voltage, varies with the number of pieces or cakes within the heating field. In an attempt to overcome these difiiculties impedance matching devices, such as tuning stubs, have been employed at various places in the prior art apparatus, and it has been necessary for a skilled operating engineer to supervise or perform the initial setting-up of the apparatus at the beginning of each run. In addition, it has been necessary to make several adjustments of the apparatus from time-to-time to minimize drying variations. Even with such devices and with care on the part of operating personnel, it has been difiicult to obtain uniform drying of materials such as, rayon cakes.

I have discovered that if the electrodes between which the material to be dried is passed are arranged so that they extend physically around a substantially centrally located inductor having an impedance substantially equal to the impedance of the electrodes at the heating energy frequency and if the electrodes are connected to such impedance by a plurality of conductors whose lengths are within a small fraction of a wave length of each other, the operation of the heating apparatus is simplified and substantially uniform drying of the material is obtained. Preferably, the centrally located impedance is the source oscillator itself, and the major portions of the conductors which connect the electrodes to the oscillator are substantially parallel, spaced discs extending outwardly from the oscillator. It has been found that with this arrangement the voltage between all portions of the electrodes is substantially the same regardless of the properties of the cakes to be dried and the number of cakes between the electrodes at any given time. Furthermore, small variations in the properties of the cakes of the type normally encountered with rayon cakes produce negligible variations in the loading of the energy source and hence, negligible variations in the voltage between the electrodes. When the oscillator itself is the centrally located impedance, greater variations in the properties of the cakes can be tolerated because such variations produce compensating changes in the oscillator frequency.

In the preferred embodiment of the invention, the

material to be dried is rotated as it is passed between,

the electrodes to obtain uniform drying. With electrodes which are substantially parallel to each other and to the axis of rotation of the material, the uniformity of drying is improved over that obtained with prior art drying ap paratus. However, better results can beobtained'by successively passing the material between electrodes which are differently disposed with respect to the material and/ or which are of different configurations. For

. example, the material may be passed first, between electrodes which are parallel to each other and the axis of rotation, then, between electrodes which are parallel to each other but perpendicular to the axis of rotation and In order to dry conventional rayon cakes in a short time, the energy input to a cake should be relatively large and may, for example, be twenty kilowatts or more. For mechanical reasons, e.g. variations in cake sizes, conveyor tolerances, etc., the practical minimum spacing between the electrodes and the cakes is about one-half inch. To produce such high energy inputs with conventional rayon cakes, which are from five to ten inches in diameter and in height, requires a voltage between the electrodes of about eight to ten thousand volts. Higher voltages are not practical because of the frequency at which arcs are produced in the equipment and hence, because of the number of production interruptions. In addition, in order to produce uniform heating of the cakes, the electric field should be substantially uniformly distributed within the cakes. I have found that with electrodes of L-shaped cross-section, the legs of the L being perpendicular and parallel respectively to the axis of a cake, a twofold increase in heat generation for a predetermined voltage or a twenty-five percent reduction in electrical stress for a predetermined rate of heat generation, as compared with plane electrodes, can be obtained.

The voltage between the electrodes and a rayon cake therebetween may be of the order of four thousand volts. Damaging arcs may form between the electrodes and the cakes and between the electrodes and other parts of the equipment, particularly when moisture condenses on such other parts. For these reasons, the cakes may be surrounded by a cylindrical shield of dielectric material and sensible heat is supplied to the parts of the equipment exposed to the high voltages so as to maintain such parts above the vapor point of water. However, damaging arcs are still formed from time-to-time, and it is important that some means be provided which will interrupt such arcs Within a few milliseconds. The preferred embodiment of the invention includes protective apparatus of my invention which will disable the heating equipment within less than three milliseconds.

It is one object of the invention to provide dielectric heating apparatus which is simple and economical to operate and which will provide uniform drying of low conductivity materials, such as rayon cakes.

Other objects and advantages of the invention will be apparent from the following detailed description of preferred embodiments of the invention, which description should be considered in connection with the accompanying drawings, in which:

FIG. 1 is a cross-sectional, side elevation view of the preferred embodiment of the heating apparatus of the mvention;

FIG. 2 is an enlarged fragmentary cross-sectional view of the condensing means forming part of the embodiment shown in FIG. 1;

FIG. 3 is a plan view, partly in cross-section, of the embodiment shown in FIG. 1;

FIG. 4 is a schematic diagram illustrating the electrical components of the embodiment shown in the preceding figures;

FIG. 5 is a circuit diagram of the oscillator forming part of the embodiment shown in the preceding figures and of the protective circuits for the oscillator;

FIG. 6 is a cross-sectional elevation view of an alternative form of support or holding means for the material to be heated;

FIG. 7 is a fragmentary perspective view of the support shown in FIG. 6;

FIG. 8 is a cross-sectional, elevation view of an alternative form of heating electrode arrangement which may be used with the embodiment shown in FIGS. 1-3;

FIG. 9 is a schematic perspective showing heating electrodes which may be employed in the embodiment shown in FIGS. 1-3; and

FIG. 10 is a schematic diagram illustrating an alternative embodiment of the invention.

In the embodiment illustrated in FIGS. l-3, the oscillator for generating the radio frequency energy is located centrally of the heating apparatus and preferably operates at a frequency of about 30 megacycles per second, al-

through it will be understood that the oscillator may be 5 operated at other radio frequencies. The oscillator comprises a centrally located vacuum tube, such as a type 5682 vacuum tube manufactured by Machlett Laboratories, Inc., Springdale, Conn, which tube can produce 200 kilowatts of radio frequency energy at 30 megacycles per second. The vacuum tube 10 comprises an anode 11 having a connecting ring 12, a grid connecting ring 13 and a pair of filament or cathode connecting rings 14 and 15. A folded first co-axial line having a grounded outer conductor 16 and an inner conductor 17 forms an inductor connected between the cathode of the tube 10 and ground. The portion of the inner conductor 17 adjacent the open end of the coaxial line is provided with a conductive extension or member 18 having an outer wall 18a and an inner wall 19. The walls 18 and 19 define an air receiving chamber 20 for purposes hereinafter described, and the inner wall 19 is electrically connected to the cathode ring 14 by means of a conductive terminal or connector 21 which is mounted on the inner wall 19 and which contacts the cathode ring 14. Filament energizing power is supplied to the filament or cathode of the tube 10 by means of the inner conductor 17 previously described and by means of the conductor 22 which is connected at one end thereof to a terminal or connector 23 which contacts the ring 15. Radio frequency bypass capacitors 24 are connected between the connectors 23 and 21.

A second co-axial line comprising an inner conductor 25 and an outer conductor 26 forms an inductor connected between the anode 11 and ground. The portion of the inner conductor 25 adjacent the open end of the second co-axial line is connected to the anode 11 by means of a connector 27 which contacts the ring 12. The annular connector 27 has a plurality of air passageways 29, 30, 31 therein for purposes hereinafter described, and the passageways 29-31 are connected to the air chamber 20 by means of a tube 32, a plurality of such tubes 32 being provided.

In an oscillator circuit, the magnitude of the impedance between the tank capacitor and the cathode and anode terminals of the oscillator tube affects the eificiency r of the oscillator. For maximum efficiency the impedance should be as small as possible, and therefore, in the embodiment shown in FIGS. l-3 the tank capacitors 33, only one of which is shown in FIG. 1, are physically as close as possible to the anode ring 12 and the cathode ring 14, the terminals of the capacitors 33 being received in sockets in the member 18 and in the connector 27 as illustrated in FIG. 1.

The inner conductor 25 is insulated from ground for direct currents and from the outer conductor 26 by means of the insulating supports 34 which are mounted at one end on a base 35 and which engage a conductive disc or plate 36 at the opposite ends, the inner conductor 25 being mounted on and secured to the disc 36. However, the inner conductor 25 and the outer conductor 26 are coupled together at radio frequencies by means of the capacitors 37 whose terminals are received in sockets in the base of the outer conductor 26 and in the disc 36 as shown in FIG. 1.

Water for cooling the anode 11 is supplied to the tube or pipe 38 through a connector 39 which is mounted on a stand comprising insulating supports 40. The anode 11 is isolated from ground at radio frequencies by means of a coil 41 wound on an insulating tube 42 and the lower end of the coil 41 which is electrically connected to the connector 39 is bypassed to ground for radio fre quencies by means of a capacitor 43.

The inner conductor 25 is cooled by means of water which is circulated through the encircling tubing 44. The terminals of the capacitors 37 are water cooled by means of Water circulating through the tubing 45-48. Cooling water from any suitable source is supplied to the tubing 4448 and to the connector 39 by conventional means through the water column formed by the insulating tubing 49 The vacuum tube is surrounded by an annular conductive ring 50 which is supported from the connector 27 by insulators 51 and which is spaced from the member 18 by insulators 52. The conductive ring 50 is connected to the grid ring 13 by means of a connector 53 and a plurality of conductive bars or tubes 54. The vacuum tube 10 is surrounded by a second, conductive, distributor ring 55 which is coupled for radio frequencies to the ring 50 by means of a plurality of capacitors 56, only one of which is shown in FIG. 1. Radio frequency driving voltage for the grid of the tube 10 is supplied by means of the capacitor 57 connected between the member 18 and the distributor ring 55.

In the design of a vacuum tube oscillator, the discharge rate of the grid blocking capacitor and the grid leak resistor combination is an important consideration and must be greater than the decrement of the tank circuit of the oscillator but not large compared to the frequency of the oscillator itself. It is common practice to make the time constant of the combination approximately equal to the time required for three to ten cycles of oscillation of the oscillator. If the time constant is too long, the oscillator will block and intermittent oscillation will result. Conversely, if it is too short, poor efficiency will result or the oscillator will fail to oscillate. With high power oscillators of the type required for the dielectric drying of materials, the grid resistor must be capable of dissipating large amounts of power, for example, as much as 8,000 watts. Accordingly, relatively large resistors of high power rating are required, and it is undesirable to locate such resistors within the oscillator housing because of the heat radiated and the voltages present, the space within the oscillator being relatively limited at the high frequencies employed. However, if such resistors are located outside of the oscillator housing, the stray or distributed capacitance of the wiring between the grid blocking capacitor and the grid resistors may, in certain cases, prevent the reduction of the size of the blocking capacitor to the value required to obtain the desired time constant. I have found that if a grid biasing circuit of the type described hereinafter in connection with FIGS. 4 and 5 is employed, the high wattage grid resistors may be located outside of the oscillator housing. As illustrated in FIG. 1, a relatively small, conductive housing 59 is mounted on the outer conductor 16 and encloses an inductor 60, a bypassing capacitor 61 and a relatively low wattage resistor 62, the inductor 60, the capacitor 61 and the resistor 62 forming part of the grid biasing circuit. These latter elements are connected to the grid ring 50 by means of the leads (not shown in FIG. 1) which extend through the interior of the inner conductor 17.

A critical problem in high frequency dielectric heating equipment is the removal of heat from glass-to-metal seals of vacuum capacitors and the oscillator vacuum tube. This heat is generated at points of very high electrical stress and arises from two separate causes. The electrical stress in the glass envelopes causes heating of the glass due to dielectric losses, and the electrical field is greatest in the glass adjacent to the glass-to-metal seal. Furthermore, all of the high frequency current which flows between the vacuum tube electrodes and the exterior terminals or connectors passes over the surface of the metal where it joins the glass producing heating of the metal. Water cooling of all the glass-to-metal seals required in an oscillator illustrated in FIG. 1 is difiicult and expensive to provide and mere circulation of air through the oscillator housing is inadequate because an air film of considerable thermal impedance clings tenaciously to all surfaces within the oscillator making it difficult to remove the heat from the glass-to-metal seals by means of low velocity air currents.

I have found that the necessary cooling of the glass-tometal seals may be obtained by impinging jets of high velocity of air on the surfaces where cooling is required. The orifices from which the air is emitted must be located in close proximity to the heated surfaces because otherwise the necessary impingement velocity will not be obtained. Since the orifices must be in close proximity to the surfaces to be cooled and since relatively high voltages are employed in the oscillator, the member or members containing the orifices should be at substantially the same electrical potential as the surfaces being cooled. I have found that by providing air passageways in the vacuum tube connectors and in the vacuum capacitor sockets, the orifices may be formed in the connectors and sockets themselves and, hence, may be close to the glassto-metal seals and at the same potential as the surfaces being cooled.

In the embodiment illustrated in FIG. 1, air at about one pound gauge pressure is supplied to the interior of the housing 59 through a conduit 63. The air flows through the housing 59, through the interior of the inner conductor 17 into the chamber 20 and from the chamber 20 into the passageways 29, 30 and 31 described above. The connector 21 is provided with a plurality of orifices 64 extending around the cathode ring 14 and which direct air at high velocity on the glass-to-metal seals between the ring 14 and the glass envelope of the tube 10. Similarly, the orifices 65 and 66 in the connectors 53 and 27 direct air at high velocity on the glassto-metal seals between the grid ring 13, the anode ring 12 and the glass envelope of the vacuum tube 10. Other orifices (not shown) in the sockets which receive the terminals of the vacuum capacitor 33 similarly direct air at high velocity on the glass-to-metal seals of the capacitors 33.

A substantially annular enclosure 67 having three walls 67a, 68 and 69 extends around the outer conductor 16, and to minimize condensation within the enclosure 67, the interior of the enclosure is heated above the boiling point of the liquid evaporated from the material to be dried by means of the heating coils 70 which may, for example, be tubes through which steam is circulated. Condensing means employed for the purposes described in my co-pending application Serial No. 601,050, filed July 30, 1956, now Patent No. 2,949,677, and comprising a plurality of walls 71, 72 and 73 (see FIGS. 1 and 2) and condensing coils 74 through which a cooling fluid is circulated extends around the upper portion of the enclosure. The condensing means is provided with an inlet 75 which opens into the upper interior portion of the enclosure for the purpose of receiving the vapor which is emitted from the material to be dried. The condensed vapor is removed from the condensing means by means of a drain 76.

Two conductive, substantially annular, co-axial heat ing electrodes 77 and 78 are mounted within the enclosure 67 andare substantiallyco-axial with the vacuum tube oscillator previously described. The heating electrodes 77 and 78 may be substantially continuous or they may be segments of conductive material. The grounded electrode 77 is supported from the walls 67a and 69 of the enclosure by spacers 79 and 80. The walls of the enclosure as well as the spacers 79- and 80 may provide the electrical connections between the electrode 77 and the outer conductor 16, but preferably, in order to provide as low an impedance path as possible, the heating electrode 77 is connected to the outer conductor 16 by means of radial strips or a perforated ring 8 1 of cooper.

-The inner heating electrode-78 is supported by a plurality of conductive tubes 82 which are mounted on feedthrough" insulators 83, supported by the outer conductor 16, and which are electrically connected to the terminals 84 secured to the distributor ring 55; The tubes 82,

7 the rings 50 and 55 and the capacitors 56 form a low impedance coupling means between the oscillator and the electrode 78.

Condensate may form on the portion 16a of the outer conductor within the enclosure 67 during the drying of the material within the enclosure 67, and if this is permitted to drain downwardly on to the insulators 813, areing may occur between the tubes 8-2 and the outer conductor 16. To aid in the removal of such condensate, an annular absorptive ring 110 of insulating material is mounted on the portion 16a of the outer conductor between the heating electrode 78 and the portion 16a. The ring 110 may be made of a material such as silicone foam, and it will absorb any condensate which drains downwardly along the portion 16a. Also, the ring 110 will be heated dielectrically because of the radio frequency voltage which exists between the heating electrode 78 and the portion 1611 so that the absorbed condensate will be revaporized and so that accumulation of the condensate on the insulators 83 will be prevented.

The heating apparatus illustrated in FIGS. l-3 is particularly adapted for the drying of cakes 8 of rayon which are filled with moisture. The cakes 85 are supported or held on stands 86 comprising a ceramic plate or disc 87 secured to ceramic insulators 88 which are secured at their opposite ends to a ceramic plate 89. The plate 89 is carried on the end of a rotatable shaft 90 which is journaled in a conveyor chain 91 which rides on the track 92. A sprocket 93 mounted on the shaft 90 is en- 'gabeable with a sprocket chain 94 mounted on the track 92 so that the stands 86 and, hence, cakes 85 are rotated continuously or intermittently as the stand is moved in a substantially circular path around the oscillator and between the heating electrodes 77 and 78. Further details of the conveyer mechanism are given in my co-pending application Serial No. 601,050, now Patent No. 2,949,677, and since the particular mechanism employed forms no part of the invention, it will not be described in further detail herein.

The enclosure 67 has an opening 95 therein (FIG. 3) permitting the continuous entrance and exit of the cakes 85 into and out of the enclosure 67. The outer heating electrode 77 terminates on opposite sides of the opening 95 so as to permit the cakes 85 to enter into and exit from between the heating electrodes 77 and 78. If desired, the inner heating electrode 78 may also terminate on opposite sides of the opening 95, but, as used herein, the expression substantially annular is intended to apply to the enclosure 67 and to the discontinuous heating electrode 77 illustrated in FIG. 3.

After the heating apparatus is electrically energized, the conveyer 91 is set in operation and the wet cakes 85 are manually loaded on the stands 86 which move into the enclosure 67. The dry cakes 85 are removed manually from the stands 86 as they exit from the opening 95. The heating apparatus of the invention is capable of drying at least 250 cylindrical rayon cakes five inchesv high and five inches in diameter per hour.

It will be noted from an examination of FIGS. l-3 that the heating electrodes 77 and 78 are closely coupled to the oscillator and are physically only a short distance from the oscillator. It has been found that with such an arrangement a variation in the number of cakes 85 between the electrodes 77 and 78 or a variation in the dielectric properties of such cakes 85 has little effect on the drying of the cakes. Furthermore, it has been found that with such vaniations the frequency of oscillation changes so that a substantially constant radio frequency voltage is present between the heating electrodes 77 and 78 at all times.

In addition, since all portions of the heating electrode I 77 and all portions of the heating electrode 78 are substantially equi-distant from the oscillator and since the heating electrodes 77 and 78 are connected to the oscillator by a plurality of low impedance conductors, the voltage between the electrodes 77 and 78 is substantially the same at all points along the path followed by the cakes even though the properties of individual cakes may differ. This means that substantially all the cakes will be subjected to the same heating field, the heating of one cake will be unefiected by the location or properties of another cake and uniform drying is obtained without adjustment of the heating apparatus.

In the preferred embodiment of the invention, the heating electrodes 77 and 78 extend in concentric circles around the oscillator, as viewed in FIG. 3, but the advantages of the invention may "be obtained if the heating electrodes extend around the oscillator in other substantially re-entrant figures, such as an ellipse, provided that all portions of an electrode are spaced from the oscillator by amounts which do not differ by more than a small fraction of a wave length at the operating frequency. For example, the amounts should not diifer'by more than approximately one-eighth of a wave length.

The spacing between the electrodes 77 and 78 is dependent upon the dimensions of the material to be heated, the voltage between the electrodes and the rate at which heat can be generated within the material. The distance between the electrodes 77 and 78 and the central oscillator is dependent upon the desired power concentration per cubic foot between the electrodes 77 and 78, the volume of material or number of cakes between the electrodes 77 and 78 at any given time, the maximum permissible voltage between the electrodes 77 and 78, the total power to be absorbed by the material to be heated in one heating apparatus and the frequency of operation. Since these factors affect the distance between the electrodes 77 and 78 and the central oscillator in different ways, a distance is selected which will provide optimum results under the given conditions. I have found, for example, that when it is desired to process approximately 250 rayon cakes five inches high and five inches in diameter per hour with an oscillator operating at approximately thirty megacycles per second, the radius of the electrode 78 should be approximately twenty-five to twenty-six inches and the radius of the electrode 77 should be approximately thirtyfour to thirty-five inches.

FIG. 4 is a schematic diagram of the electrical components of the embodiment shown in FIGS. 1-3 and of the rayon cakes 85 and their supporting or holding stands 86. The components shown in FIG. 4 and which have been described above are identified by the same reference numerals.

FIG. 4 illustrates the electrodes of the vacuum tube 10 including, an anode 11, a control grid 96 and a filament or cathode 97, the electrodes being enclosed in a glass envelope 98. For the reasons set forth above, the grid 96 is connected to ground through a radio frequency choke 99, a lead 100 which passes through the interior of the inner conductor 17, the resistor 62 connected in parallel with the choke 60, a line 101, and a plurality of parallel connected, high wattage resistors 102 which are located exteriorly of the oscillator housing. One end of the choke 99 is bypassed to the cathode 97 at radio frequencies by means of a capacitor 103 and one end of the choke 60 is bypassed to ground by the capacitor 61.

The bias required for the grid 96, although essentially constant, contains a low amplitude modulation varying in frequency from Zero to at least ten megacycles. In order to maintain proper bias on the grid 96, the biasing voltage should follow the modulated voltage and it is for this reason that when the biasing resistors, such as the resistors 102, are located outside of the oscillator housing and are connected to the grid of the oscillator tube by a line having substantial capacitance, additional circuit components in positions substantially unaffected by such capacitance should be provided. The choke 60 provides a low resistance path for direct currents and components of less than 1000 cycles per second and has a relatively high impedance for components of higher frequencies.

The resistor 62 has a value of resistance which is relatively low compared to the impedance of the choke 60 at such higher frequencies and the capacitor 61 has a relatively low impedance at such frequencies. The choke 6%) has a relatively high impedance at frequencies as low as ten kilocycles per second and the capacitor 61 has a relatively low impedance at frequencies as low as ten kilocycles per second. Therefore, modulation currents having frequency components of from ten kilocycles to several megacycles flow largely through resistor 62 and capacitor 51 dissipating the energy stored in blocking capacitor 56 and thereby maintaining the proper negative bias on the grid of tube 10. It will be noted that although the choke 99 has a relatively high impedance and the capacitor 1% has a relatively low impedance at the frequency of oscillation, they have relatively low and relatively high impedances, respectively, at the frequencies of the modulating component.

With an oscillator of the type described and a frequency of oscillation of thirty megacycles, proper operation of the oscillator may be obtained when the components of the grid biasing circuit have the following values:

Resistor 62-500 ohms, 200 Watts.

Combined parallel resistance and wattage of resistors 102-500 ohms, 8 kilowatts.

Choke 60-30 millihenries.

Choke 99-self-resonant at 30 megacycles per second.

Capacitor 6 110G0 micromicrofarads.

Capacitor 103-50 micromicrofarads.

For the purpose of maintaining the distributor ring 55, and hence, the heating electrode 78, at ground potential for direct currents, the oscillator includes a lead 104 which extends into the housing 59 and which is connected to the distributor ring 55 through a radio frequency choke 105. One end of the choke 105 is bypassed for radio frequencies to the cathode 97 by way of the member 18 by a capacitor 106. Although the lead may be grounded at other points, it preferably extends into the housing 59 for convenience in checking the operation of the oscillator without disassembling the oscillator housing. For example, the existence of short circuits in the capacitors 56, 57, 103' and 106 can be determined by applying test voltages between leads 100 and 104 and between the leads 1% and 104 and ground.

For the purpose of measuring the filament energizing voltage, the oscillator may be provided with a pair of leads 1d? and 108 which is passed through the inner conductor 17 and which are connected to a voltmeter 109.

FIG. is a circuit diagram of the oscillator shown in the preceding figures and the circuit of the invention for protecting the oscillator and associated circuits against damage due to arcing within the oscillator. It will be noted from an examination of FIG. 5 that the oscillator circuit is of the Colpitts type, and therefore, its operation is readily apparent to those skilled in the art.

It has been found that during the operation of the heat-- ing apparatus previously described, damaging arcs may occur from time to time between the heating electrodes and the material being dried, between the electrodes and other parts within the enclosure 67 and between various parts of the oscillator itself. Such arcs cause an increase in the dissipation of radio frequency energy and are accompanied by a rapid drop in the radio frequency voltage within the oscillator. It has been found that in a dryer of the type described, such arcs produce changes of radio frequency voltage which are more rapid than the changes in radio frequency voltage produced by any other cause of loading variation. It is important to extinguish such arcs Within a few milliseconds, and I have invented a protective circuit which will respond within such a short period of time to rapid rates of change of radio frequency voltage caused by arcs but which will not respond to changes of radio frequency voltage caused by low frequency modulation of the radio frequency voltage produced by the low frequency power supply and to the relatively low frequency modulation of the radio frequency voltage which occurs during the time that the oscillator is being put into operation.

In the preferred embodiment of the protective circuit, the anode 11 of the oscillator tube 10 is connected to the positive terminal of a direct current supply 111 through a surge reactor 112 and through a parallel network comprising a surge capacitor 113, a current limiting resistor 114 and the contacts 115 of a vacuum switch having an envelope 116. The voltage between the positive terminal of the DC. supply 111 and ground may, for example, be of the order of fifteen to twenty thousand volts. The anode 117 of a high voltage gas tube 118, such as a type 5948 hydrogen thyratron having a high current capacity is connected to the junction point of the reactor 112 and the resistor 114. The cathode 119 of the tube 118 is connected to ground through the operating coil 120 of the vacuum switch and through the operating coil 121 of a relay whose contacts 122 are connected in series between the alternating current source 123 and the input of the direct current supply 111. The cathode circuit of the tube 118 may optionally include a biasing resistor 124. The grid 125 of the tube 118 is connected to the cathode 126 of a second gas tube or thyratron 127, the cathode 126 being connected to ground through a resistor 128. The anode 129 is connected to the positive terminal of a low voltage direct current supply 131 comprising a capacitor 130a through a current limiting resistor 13%. Preferably, the impedance of the supply 130 and the value of the capacitor 130a are such that When the gas tube 127 conducts, as hereinafter described, the capacitor 130a discharges and the potential at the anode 129 becomes less than the ionizing potential of the tube 127, causing the tube 127 to become nonconducting again.

The control grid 131 is connected to ground through a biasing network comprising resistors 132-434 and a capacitor 135, the terminal 136 being connected to the negative terminal of a low voltage direct current supply (not shown) which provide a biasing potential sufficient to maintain the tube 127 normally non-conducting.

The direct current voltages applied to the gas tubes 118 and 127 are such that these tubes normally are nonconducting, but when a positive pulse is applied to the grid 131 through the capacitor 137, both of the gas tubes 118 and 127 become conducting. The gas tube 118 will become conducting in less than a millisecond after the positive pulse is applied to the grid 131 and will substantially short circuit the junction point of the reactor 1'12 and the resistor 1 14 to ground within this time thereby removing the anode voltage from the oscillator tube 10.

If the anode 117 were connected to a low voltage supply, the current produced through the operating coil 12%) of the vacuum switch when the tube 118 becomes conducting would cause the contacts 115 of the vacuum switch to open in about 20-30 milliseconds after the tube 118 becomes conducting. During this time the tube 118 substantially short circuits the output of the direct current supply 111 and transient currents and voltages may be produced which may damage the direct current supply 111. However, since the anode 117 is connected to the high voltage supply 111 and since a very large current results when the tube 118 becomes conducting, a strong magnetic field is produced by the coil 120 in a fraction of a millisecond which is suflicient to open the contacts 115 within two to three milliseconds. When the contacts 115 are opened, the resistor 114 is connected in series between the positive terminal of the DC. supply 111 and the anode 117 of the tube 118 and the undesirable transient currents and voltages are thereby limited in duration to approximately two or three milliseconds.

Shortly after the vacuum switch operates, as described 11 above, the contacts 122 are opened by virtue of the current flowing through the coil 121 so that the DC. supply 111 becomes de-energized. At this time the high voltage is interrupted, and the tube 118 becomes non-conducting. The contacts 122 remain open until manually or automatically reset.

The circuit within the dotted rectangle 138 is a well known type of one-cycle, trigger circuit, known as a Sohmitt circuit, which supplies a positive output pulse when a positive voltage above a predetermined value is applied to the grid of the vacuum tube 140. The grid 139 is connected to the output of a rectifying, filtering and differentiating circuit and a positive voltage of sufficient magnitude to operate the trigger circuit 138 is developed across the resistor 141 when a rate of radio frequency energy decrease which normally accompanies the occurrence of a damaging arc in the oscillator occurs. However, radio frequency energy changes of the type encountered during the energization of the high frequency oscillator or which are caused by the alternating current modulation of the radio frequency energy at the frequency of the source 123, for example 360 cycles per second, will not produce a positive voltage of sufiicient magnitude across the resistor 141 to trigger the circuit 138.

, The rectifying, filtering and differentiating network comprises resistors 141-144, capacitors 145 and 146, a radio frequency choke 147 and a rectifier 148 which is poled so as to produce a negative voltage at the junction point of the resistors 142 and 143 and the choke 147. Radio frequency energy from the high frequency oscillator comprising the vacuum tube is supplied to the input of the circuit by means of the coaxial line 149 which terminates at the end remote from the circuit in a coupling loop 150 which may be coupled to any por tion of the high frequency oscillator which will supply high frequency energy to the rectifying, filtering and differentiating circuit of sufiicient amplitude to produce the desired positive voltage at the grid 139. The loop 150 may, for example, be coupled to the co-axial line which is connected to the anode 11 of the vacuum tube 10 as shown in FIG. 4.

The time constants and magnitudes of the elements of the rectifying, filtering and differentiating circuit are selected in a manner well known to those skilled in the art so as to provide operation of the protective circuit when the magnitude of change and the rate of change of the radio frequency voltage exceed first predetermined values and are less than second predetermined values. It has been found that with the embodiment previously described the desired operation may be obtained if the elements have the following values:

Resistor 141-12,000 ohms Resistor 142--25,000 ohms Resistor 143-3,000 ohms Resistor 144-73 ohms Capacitor 145-20 micromicrofarads Capacitor 146l,500 micromicrofarads Choke 147self-resonant at megacycles per second.

When the elements of the circuit have the values set forth above, the loop 115 may be so oriented and placed with respect to the radio frequency field of the oscillator as to produce a steady state voltage of approximately fortyfive R.M.S. volts across the resistor 144. With the trigger circuit 138 adjusted to operate with a voltage of three volts across the resistor 141, the voltage being positive at the end of the resistor 141 connected to the grid 139, it has been found that the protective circuit will not operate when the radio frequency voltage varies about eight percent in magnitude because of modulation thereof at 360 cycles per second by the power supply or because of the transients produced when the oscillator is initially energized. In addition, because of the capacitor 145, the resistor 143 and the choke 147, the protective circuit will not be operated by the radio frequency voltage itself the resistor 114, and the vacuum switch comprising the 7 portion 152.

operating coil 120 and the contacts 115 may be omitted.

Although the stand or holding means 86 for the cakes 85, shown in FIG. 1, is satisfactory for transporting the cakes between the heating electrodes 77 and 78, care must be taken to place the cakes 85 centrally of the stand and at times the cakes 85 tend to move sidewise with respect to the stand 86 during the time that they are transported between the heating electrodes. In order to provide uniform drying of the cakes 85 and to prevent arcing between the heating electrodes 77 and 78 and the cakes 85, the cakes 85 should be spaced from the heating electrodes. The stand 151, shown in FIGS. 6 and 7, simplifies the loading of the cakes 85 on the stand and holds such cakes 85 in place during the drying cycle.

Since the stand 151 is subjected to the radio frequency field and since losses in the stand should be kept to a minimum, the stand 151 preferably is made of a low loss ceramic material, such as steatite, and comprises a lower tubular portion 152 and an upper flange portion 153. The lower tubular portion 152 frictionally engages a metal insert 154 which is held on the shaft or spindle by means of a bolt 155. A removable locator 156, made of a low loss insulating material such as tetrafluoroethylene, is mounted on top of the flange portion 153 and has a portion which extends within the bore of the tubular The locator 156 maintains the cake 85 in the correct position on the top of the stand 151 and since the locator 156 is made of low loss material and has a relatively small volume, the locator 156 has little effect on the electric field through the cake 85.

In the embodiment previously described, the heating electrodes 77 and 78 are vertical and extend in planes substantially parallel to the axes of the cakes 85. When the heating electrodes are so disposed, a substantially uniform electric field is obtained throughout the cakes 85. With an operating frequency of 30 megacycles per second and an energy input of the order of 20 kilowatts per wet cake, each cake 85 has relatively low impedance. The high energy input is desirable to obtain a fast drying rate, and with vertical heating electrodes and a spacing of about one-half inch between the electrodes and the cakes 85 a voltage between the heating electrodes 77 and 78 of about 8,00010,000 volts is required to obtain energy inputs of the order of 20 kilowatts. Under these conditions the voltage between diametrically opposite points on the cakes 85 is of the order of 2,000 volts.

With the vertical heating electrodes previously described, the current flux density is relatively high. If the vertical electrodes are merely replaced by horizontal heating electrodes, there is substantially no change in the current flux density, and in addition, the electric field through the cakes is less uniform. Heating electrodes extending transversely to the axis of the cake and substantially parallel to a line drawn between the diagonally opposite corners of the cake, as viewed in cross-section produce an intense concentration of heat at the corners of the cake, and therefore, such electrodes do not produce uniform drying of the cakes.

If the heating electrodes are shaped as shown in FIG. 8, it is possible, with a given voltage, to obtain a increase in the amount of heat generated in a cake 85 as compared with either vertical or horizontal plane electrodes. Furthermore, a substantially uniform electric field through the cake 85, as indicated by the dotted lines 157, may be obtained, and hence, with such electrodes it is possible to obtain uniform drying of the cake 85. The heating electrodes 1'58 and 159, shown in FIG. 8, have vertical portions 160 and 161 which extend parallel to the axes of the cakes 85 and horizontal portions 162 and 163 which extend horizontally and substantially perpendicular to the axes of the cakes 85. With the use of heating electrodes shaped in this manner, the current flux density for a given voltage may be reduced as compared to plane, vertical or horizontal electrodes. Furthermore, if no increase in the amount of heat generation is desired, a reduction in the electrical stress or voltage of 25% may be obtained for the same rate of heat generation in the cake 85.

When the material to be heated and dried is rayon wound in cakes, small fibres may extend from the cakes which may brush against the heating electrodes or produce electrical stresses which promote the formation of damaging arcs. Such arcs may be reduced in frequency and the number of arcs terminating at the surface of the cake 85 may be reduced by surrounding the cake 85 by a cylinder or sleeve 164 of insulating material. Since the sleeve 164 also is in the electric field, it should be formed of a low loss material, and it has been found that a sleeve formed from glass cloth impregnated with a silicone resin provides satisfactory results. The sleeve 164 may be se-.

cured to the flange portion 153 of the stand 151 in any desired manner and may, for example, be adhesively secured thereto by means of a silicone resin. Alternatively, the stand 151 including the flange portion 153 and the sleeve 164 may be an integral unit molded from fiber glass cloth bonded with a silicone resin.

Although good results are obtained when only one type of heating electrodes extend around the centrally located oscillator, it may in some cases be desirable to employ one or more series of differently disposed and/or shaped electrodes around the periphery of the oscillator. For example, plane vertical electrodes extending parallel to the axis of the cake 85 cause removal of moisture from the peripheral surface of the cake at a rate greater than the rate at which it is removed from the center thereof. Plane horizontal electrodes extending substantially perpendicular to the axis of the cake 85 cause removal of moisture from the bore of the cake at a greater rate than the rate at which the moisture is removed from the outer periphery thereof. Electrodes of the type illustrated in FIG. 8 provide a combination of such results. However, if the electrodes are arranged in a series as illustrated in FIG. 9, and if the cakes 85 are transported successively between such electrodes, then the combined effects of the different electrodes are obtained so that all parts of the cakes 85 receive the same treatment. The arrangement of electrodes illustrated diagrammatically in FIG. 9 comprises a pair of vertical electrodes 77 and 78 of the type illustrated in FIGS. 1 and 3, a pair of horizontal electrodes 165 and 166 and a pair of L-shaped electrodes 158 and 159 of the type illustrated in FIG. 8.

Although heating apparatus of the type illustrated in FIGS. 1-3 in which the oscillator itself is located centrally of the heating electrodes provides the best results because the heating energy voltage is substantially the same between all parts of the heating electrodes and because the heating of the material to be dried is substantially unaffected by the normal loading changes encountered between the electrodes, the former advantage can be obtained with apparatus of the type illustrated in FIG. 10 in which the high frequency oscillator is not located within the heating electrodes 77 and 78. The heating apparatus, illustrated schematically in FIG. 10, comprises a high frequency source 167 which includes the high frequency oscillator and which may be located remotely from the conductive central housing 168. The housing 168 encloses an inductor 169, which is connected or coupled at its opposite ends by means of conductive discs, such as discs of copper, to the electrodes 77 and 78. Instead of copper discs a plurality of radially extending bars or strips may be employed as long as the ends of the inductor 169 are coupled to the heating electrodes 77 and 78 by conductors of substantial total area which provide low impedance paths between the ends of the inductor 169 and the electrodes 77 and 78. The inductor 169 has inductance sufiicient to provide, in combination with the conductors 170 and 171 and the heating electrodes 77 and 78 a parallel resonant circuit which is resonant at the operating frequency of the source 167. The inductor 169 is adjustable as indicated so as to permit tuning of the system to resonance, and the source 167 is connected to the inductor 169 by means of a transmission line 172. Although the transmission line 172 is shown connected to the inductor 169 by an adjustable tap 173, it will be understood that the transmission line 172 may be coupled to the inductor 169 in any other well known manner.

Having thus described my invention with particular reference to the preferred form thereof and having shown and described certain modifications, it will be obvious to those skilled in the art to which the invention pertains, after understanding my invention, that various changes and other modifications may be made therein without departing from the spirit and scope of my invention, as defined by the claims appended thereto.

What is claimed as new and desired to be secured by Letters Patent is:

1. High frequency heating apparatus comprising: a radio frequency oscillator comprising a vacuum tube having a plurality of electrodes, and resonant circuit means coupled in circuit with said electrodes for generating radio frequency energy; an enclosure extending around said oscillator; a plurality of pairs of spaced heating electrodes mounted Within said enclosure and extending around said oscillator; material holding means mounted between said heating electrodes, said pairs of heating electrodes having surfaces facing said holding means, said surfaces of one pair of said pairs of heating electrodes being disposed with respect to said'holding means at an angle which differs from the angle at which said surfaces of another pair of said pairs of heating electrodes are disposed with respect to said holding means; and low impedance means coupling said heating electrodes to said oscillator.

2. High frequency heating apparatus comprising: a high frequency oscillator comprising a vacuum tube having a plurality of electrodes, and resonant circuit means coupled in circuit with said electrodes for generating radio frequency energy; an enclosure extending around said oscillator; a first pair of spaced heating electrodes mounted in said enclosure; a second pair of spaced heating electrodes mounted in said enclosure and having portions extending transversely to said first pair of heating electrodes, said second pair of heating electrodes being spaced from said first pair of heating electrodes in a direction extending around said oscillator; means for successively passing material to be heated between said first and said second pair of heating electrodes; and low impedance means coupling said heating electrodes to said oscillator.

3. Dielectric heating apparatus comprising: a radio frequency oscillator comprising a vacuum tube having at least a cathode, an anode and a grid, a first, tuned, co-axial line having inner and outer conductors and having an open end, means connecting the portion of said inner conductor adjacent said open end to said cathode, a second, tuned, co-axial line having inner and outer conductors and having an open end, said second line being mounted co-axially with said first line and with the open end thereof facing the open end of said first line, means connecting the portion of said inner conductor of said second line adjacentthe open end of said second line to said anode and the portion of said outer conductor of said first line adjacent the open end of said first line being connected to the portion of said outer conductor of said second line adjacent the open end of said second line, and a capacitor mounted adjacent said cathode and said anode and connected therebetween; a substantially annular enclosure extending around one of said outer conductors; a first substantially annular electrode mounted within said enclosure and extending around one of said outer conductors; a conveyer extending around said electrode, said enclosure having an opening therein permitting access to said conveyer; a second electrode extending around said conveyer except for the portion thereof adjacent said opening in said enclosure; low impedance means connecting one of said electrodes to one of said outer conductors; and low impedance means connecting the other of said electrodes to said grid.

4. Dielectric heating apparatus comprising: a radio frequency oscillator comprising a vacuum tube having at least a cathode, an anode and a grid, a first, tuned, co-axial line having inner and outer conductors and having open and closed ends, means connecting the portion of said inner conductor adjacent said open end to said cathode, a second, tuned, co-axial line having inner and outer conductors and having open and closed ends, said second line being mounted co-axially with said first line and with the open end thereof facing the open end of said first line, means connecting the portion of said innet conductor of said second line adjacent the open end of said second line to said anode and the portion of said outer conductor of said first line adjacent the open end of said first line being connected to the portion of said outer conductor of said second line adjacent the open end of said second line, a capacitor mounted adjacent said cathode and said anode and connected therebetween, a capacitor mounted adjacent said cathode and said grid and connected therebetween, a conductive ring extending around said vacuum tube and mounted within one of said outer conductors, and means connecting said ring to said grid; a substantially annular enclosure extending around one of said outer conductors; a first substantially annular electrode mounted within said enclosure and extending around one of said outer conductors; a conveyer extending around said electrode, said enclosure having an opening therein permitting access to said conveyer; a second electrode extending around said conveyer except for the portion thereof adjacent said opening in said enclosure; low impedance means connecting one of said electrodes to one of said outer conductors; and low impedance means connecting the other of said electrodes to said ring.

5. Dielectric heating apparatus comprising: a radio frequency oscillator comprising a vacuum tube having at least a cathode, an anode and a grid, a first, tuned, coaxial line having inner and outer conductors and having open and closed ends, means connecting the portion of said inner conductor adjacent said open end to said cathode, a second, tuned, co-axial line having inner and outer conductors and having open and closed ends, said second line being mounted co-axially with said first line and with the open end thereof facing the open end of said first line, means connecting the portion of said inner conductor of said second line adjacent the open end of said second line to said anode and the portion of said outer conductor of said first line adjacent the open end of said first line being connected to the portion of said outer conductor of said second line adjacent the open end of said second line, a capacitor mounted adjacent said cathode and said anode and connected therebetween, a capacitor mounted adjacent said cathode and said grid and connected therebetween, a conductive ring extending around said vacuumtube and mounted within one of said outer conductors, and means connecting said ring to said grid; a substantially annular enclosure extending around one of said outer conductors; vapor condensing means having an inlet communicating with the upper interior portion of said enclosure; a first substantially annular electrode mounted within said enclosure and extending around one of said outer conductors; low impedance means connecting said electrode to said ring; a conveyer extending around said electrode, said enclosure having an opening therein permitting access to said conveyer; a second electrode extending around said conveyer except for the portion thereof adjacent said opening in said enclosure; and low impedance means connecting said second electrode to one of said outer conductors.

6. Dielectric heating apparatus comprising: a radio frequency oscillator comprising a vacuum tube having at least a cathode, an anode and a grid, a first, tuned, co-axial line having inner and outer conductors and having open and closed ends, means connecting the portion of said inner conductor adjacent said open end to said cathode, a second, tuned, co-axial line having inner and outer conductors and having open and closed ends, said second line being mounted co-axially with said first line and with the open end thereof facing the open end of said first line, means connecting the portion of said inner conductor of said second line adjacent the open end of said second line to said anode and the portion of said outer conductor of said first line adjacent the open end of said first line being connected to the portion of said outer condutor of said second line adjacent the open end of said second line, a plurality of capacitors mounted ad jacent said cathode and said anode and connected therebetween, a plurality of capacitors mounted adjacent said cathode and said grid and connected therebetween, a housing mounted on the outer conductor of said first line, a first resistor mounted within said housing, means connecting one end of said first resistor to said grid, a second resistor mounted outside of and remote from said housing, means connecting the other end of said first resistor to one end of said second resistor, means connecting the other end of said second resistor to one of said outer conductors, a conductive ring extending around said vacuum tube and mounted within one of said outer conductors, and means connecting said ring to said grid; a substantially annular enclosure extending around one of said outer conductors; vapor condensing means having an inlet communicating with the upper interior portion of said enclosure; means for heating the interior of said enclosure; a first substantially annular electrode mounted within said enclosure and extending around one of said outer conductors, said electrode having a portion extending substantially parallel to the axes of said lines; low impedance means connecting said electrode to said ring; a substantially annular conveyer extending around said electrode; a second substantially annular electrode extending around said conveyor and having a portion extend-ing substantially parallel to the axes of said lines, said second electrode and said enclosure having aligned openings therein permitting access to said conveyer; and low impedance means connecting said second electrode to one of said outer conductors.

7. High frequency heating apparatus for heating dielectric material comprising a radio frequency oscillator, and a pair of spacedheating electrodes electrically coupled to said oscillator, each said electrode having a pair of electrically connected portions, one of said portions of one said electrode extending substantially parallel to one portion of the other of said electrodes and the other of said portions of said electrodes extending transversely to said one portion thereof and on opposite sides of the space between said one portion of said electrodes.

8. High frequency heating apparatus for heating dielectric material comprising a radio frequency oscillator, and a pair of spaced heating electrodes electrically coupled to said oscillator, each of said electrodes having a pair of electrically connected portions, one of said portions extending substantially perpendicular to the other of said portions and said one of said portions of one electrode extending substantially parallel to said one portion of the other of said electrodes and the other of said portions of said one electrode extending substantially parallel to said other portion of said other electrode, both of said other portions extending intermediate said one portion.

leg thereof on one side of said holding; means and with the otherleg' thereofextending over said holdingmeans and the, other of said electrodes" being" disposed with one leg thereofon'the opposite sideof'said holding means and with the other; leg thereofi below? said holding means.

l0. High-' frequency heating: apparatus comprising a radio fiequency oscillator, a pair of' spaced heating electrodes electrically coupled to said oscillator, and a rotatable material holding stand disposed intermediate said electrodes, said stand comprising a member of insulating material having a lower portion and a disc-like upper flange portion extending substantially perpendicularly to the axis of rotation of said stand, said upper portion having a centrally disposed opening therein, and a locator of insulating material mounted in said opening and on said upper portion, said locator comprising a pair of intersecting plate-like members mounted with the line of intersection extending substantially coincident with said axis of rotation.

11. High frequency heating apparatus for heating dielectric material comprising a radio frequency oscillator, means for holding material to be heated, a pair of spaced heating electrodes disposed on opposite sides of said holding means and electrically coupled to said oscillator, and fluid absorbing means made of insulating material disposed adjacent one of said electrodes and outside of the space between said electrodes but between said one electrode and a point having a radio frequency potential different from that of said one electrode for absorbing and re-evaporating fluid emitted from said material to be heated.

12. High frequency heating apparatus comprising: a radio frequency oscillator comprising a vacuum tube having a plurality of electrodes, and resonant circuit means coupled in circuit with said electrodes for generating radio frequency energy; a substantially annular enclosure extending around said oscillator; a pair of spaced heating electrodes mounted within said enclosure and extending around said oscillator; material holding means mounted between said electrodes; low impedance means coupling said heating electrodes to said oscillator; and fluid absorbing means mounted between one of said electrodes and said enclosure but outside of the space between said electrodes for absorbing and re-evaporating fluid emitted by said material.

13. High frequency heating apparatus comprising: a radio frequency oscillator comprising a vacuum tube having at least a cathode electrode, an anode electrode and a control electrode, a housing for said oscillator comprising a pair of tuned, co-axial lines mounted co-axially with each other, means coupling one of said lines to one of said electrodes, means coupling the other of said lines to another of said electrodes, a radio frequency first choke, a second choke having a value of inductance larger than that of said first choke and a first resistor connected in series in the order named and between said grid and said cathode electrodes, a radio frequency first bypassing capacitor connected between the junction point of said first and second chokes and said cathode electrode, a second bypassing capacitor having a relatively low impedance at frequencies substantially lower than said radio frequency connected between the junction point of said resistor and said second choke and said cathode electrode, and a second resistor having an impedance which is low relative to the impedance of said second choke at frequencies substantially lower than said radio frequency connected in parallel with said second choke, said chokes, said second resistor and said firstbypass'ing capacitor" b'eing mounted within said housing: and saidfirst resistor being; mounted outside of said housing; a substantially annular enclosure mounted co-axially'with and'extending around one 'ofsaid lines a pair of spaced heating electrodes" mounted within said enclosure and extending'aroundthe'major portion of the periphery ofoneof said'lines; material holdingmeans mounted between" said electrodes; and low impedance means coupling" said heating electrodes to said oscillator.

14i High frequency heating apparatus comprising; a radio frequency oscillator and a pair of spaced heating electrodeselectrically 'coupledtosaidoscillator, said"oscillator comprising a" vacuum tube having a plurality of electrodes including a" cathode'and'a grid, resonant circuit means coupled in circuit with said tube electrodes for generating radio frequency energy and means for biasing said tube comprising first and second chokes and a first resistor connected in series in the order named and between said cathode and said grid, one end of said first choke being connected to said grid, a first capacitor connected between the other end of said first choke and said cathode, a second capacitor connected between the junction point of said second choke and said first resistor and said cathode, and a second resistor connected between the ends of said second choke, said first choke and said first capacitor having relatively high and low impedances respectively at said nadio frequency, said second choke having a relatively high impedance at predetermined frequencies substantially lower than said radio frequency but having a high impedance at still lower frequencies, said second capacitor having a relatively low impedance at said predetermined frequencies, and said second resistor having a relatively low impedance at said predetermined frequencies.

15. A radio frequency oscillator comprising a vacuum tube having a plurality of electrodes including a cathode and a grid, resonant circuit means coupled in circuit with said tube electrodes for generating radio frequency energy and means for biasing said tube comprising first and second chokes and a first resistor connected in series in the order named and between said cathode and said grid, one end of said first choke being connected to said grid, a first capacitor connected between the other end of said first choke and said cathode, a second capacitor connected between the junction point of said second choke and said first resistor and said cathode, and a second resistor connected between the ends of said second choke, said first choke and said first capacitor having relatively high and low impedances respectively at said radio frequency, said second choke having a relatively high impedance at predetermined frequencies substantially lower than said radio frequency but having a high impedance at still lower frequencies, said second capacitor having a relatively low impedance at said predetermined frequencies, and said second resistor having a relatively low impedance at said predetermined frequencies.

16. A radio frequency oscillator comprising a vacuum tube having a plurality of electrodes including a cathode and a grid, resonant circuit means coupled in circuit with said tube electrodes for generating radio frequency energy and means for biasing said tube comprising a radio frequency first choke, a second choke having an inductance which is higher than that of said first choke, and a first resistor connected in series in the order named and between said cathode and said grid, one end of said first choke being connected to said grid, a radio frequency bypassing, first capacitor connected between the other end of said first choke and said cathode, a second capacitor having a capacitance which is larger than that of said first capacitor connected between the junction point of said second choke and said first resistor and said cathode, and a second resistor connected between the ends of said second choke.

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